Magnetic compasses are employed in a variety of position and/or orientation sensing systems. One or more magnetometers may be used in the compass. Calibration of the magnetic compass is relatively straightforward in a controlled test environment where a calibration rotation can be precisely controlled. However, when the magnetic compass is deployed in the field, the accuracy of the magnetic compass may be degraded.
The device in which the magnetic compass resides may contain metals or other magnetic materials which alter the magnetic field in the vicinity of the magnetic compass. For example, the magnetic compass may be installed in a vehicle, such as an aircraft, ship, or military tank. Thus, if the magnetic compass is calibrated prior to installation in a device and/or deployment in the field, the magnetic fields altered by the nearby metals or the other magnetic materials may introduce error into the magnetic compass bearing readings.
If the magnetic compass is installed in a portable device, external metals or other magnetic materials which come into proximity of the magnetic compass may further alter the magnetic field in the vicinity of the magnetic compass. That is, the magnetic fields in the vicinity of the magnetic compass may change as a device or a vehicle in which the magnetic compass is deployed is moved. Accordingly, moving the magnetic compass into the vicinity of external metals or other magnetic materials may introduce error into the magnetic compass bearing readings.
For example, the magnetic compass may be installed in a portable device that may be conveniently carried on a person, such as when attached to clothing or placed in a back pack. Metal camping gear or munitions carried by the person may alter the magnetic field in the vicinity of the magnetic compass. If the person is a soldier, the person's metal protective headgear (e.g., a helmet) may further alter the magnetic field in the vicinity of the magnetic compass. When the headgear is put on or removed, and/or as munitions are added or used, the magnetic field in the vicinity of the magnetic compass may change. Thus, the accuracy of the magnetic compass may be changed.
Accordingly, it may be desirable to recalibrate the magnetic compass on a periodic basis, prior to use, during use, and/or after some event. For example, recalibration may be desirable after batteries containing ferromagnetic materials (casing or leads) have been changed. As another example, recalibration may be desirable after the soldier has geared up for field deployment. Thus, it is desirable to recalibrate in the field so that the magnetic compass provides accurate bearing information for the particular location and/or environment that it is to be used.
Often, one or more gyroscopes (and/or accelerometers) will be deployed with the magnetic compass. Gyroscope may also be inaccurate during use in the field. For example, gyroscopes are subject to drift rates. Accordingly, it may also be desirable to field calibrate the gyroscopes along with the recalibration of the magnetic compass.